1. Field of the Invention
The invention relates to rolling bearings.
2. Description of the Related Art
Rolling bearings known in the related art support rotary shafts of various mechanisms. Rolling bearings require high reliability in performing their functions, This means that rolling bearings preferably do not cause seizure, for example. To prevent seizure, rolling bearings are lubricated with grease having high lubricating performance. Such rolling bearings are increasingly being used in high speed rotation applications. Japanese Patent Application Publication No. 2010-164122 (JP 2010-164122 A), for example, discloses a bearing whose inner portion retains, in advance, grease for lubrication.
FIG. 5 is a cross-sectional view of examples of conventional rolling bearings 90. Referring to FIG. 5, a rotary shaft 99 is oriented longitudinally (or vertically), and a center line C0 of each rolling bearing 90 extends vertically. Each rolling bearing 90 is an angular contact ball hearing and will thus be referred to as an “angular contact ball bearing 90”. Each angular contact ball bearing 90 includes an inner ring 91, an outer ring 92, and balls 94 each in contact with the inner ring 91 and the outer ring 92 at a predetermined angle (or contact angle).
The angular contact ball bearings 90 are often used in a pair. A lower angular contact ball bearing 90A and an upper angular contact ball bearing 90B are often attached to a bearing housing 100 etc. so that the contact angle of each ball 94 of the lower angular contact ball bearing 90A and the contact angle of each ball 94 of the upper angular contact ball bearing 90B are oriented in opposite directions. This enables the lower angular contact ball bearing 90A and the upper angular contact ball bearing 90B to support axial loads in both of the directions.
Grease is retained in the inner portion of each angular contact ball bearing 90. Specifically, grease is retained in an annular space 93 defined between the inner ring 91 and the outer ring 92 of each angular contact ball bearing 90. Each angular contact ball bearing 90 is provided with seals 97 and 98 on both axial sides of the annular space 93. To enable high speed rotation, the seals 97 and 98 of each angular contact ball bearing 90 illustrated in FIG. 5 are labyrinth seals (or non-contact seals). The seals 97 and 98 respectively define labyrinth clearances 97a and 98a with the inner ring 91. Thus, the grease retained in the annular space 93 is prevented from leaking to the outside.
Rotation of the inner ring 91 of each angular contact ball bearing 90 causes the grease in the annular space 93 to move in the axial direction under centrifugal force because a pair of shoulders 95 and 96 of the inner ring 91 have different outside diameters. In particular, when each angular contact ball bearing 90 rotates at a high speed, this movement is pronounced. Rotation of the inner ring 91 of the lower angular contact ball bearing 90A in FIG. 5 causes the grease in the annular space 93 to move from a first axial side to a second axial side, i.e., from bottom to top, under centrifugal force. In contrast, rotation of the inner ring 91 of the upper angular contact ball bearing 90B in FIG. 5 causes the grease in the annular space 93 to move from the second axial side to the first axial side, i.e., from top to bottom, under centrifugal force.
When the rotation of the inner ring 91 of the lower angular contact ball bearing 90A is stopped, the centrifugal force mentioned above ceases. This causes the grease in the annular space 93 of the lower angular contact ball bearing 90A to move downward under gravity (or its own weight). The flow of the grease along the outer peripheral surface of the shoulder 96 of the inner ring 91 may result in passage of the grease through the labyrinth clearance 97a and leakage of the grease to the outside. The leakage of the grease may cause lubrication failure and lead to defective conditions, such as, seizure and abnormal wear.
When the rotation of the inner ring 91 of the upper angular contact ball bearing 90B is stopped, the centrifugal force ceases. This causes the grease in the annular space 93 of the upper angular contact ball bearing 90B to move downward under gravity (or its own weight). As indicated by the arrow A in FIG. 6, the grease flows along the outer peripheral surface of the shoulder 95 of the inner ring 91, so that the grease leaves an edge 101 at the end of the shoulder 95 of the inner ring 91 and reaches a lateral surface 98b of a lip 98c of the seal 98. As indicated by the arrow A1 in FIG. 6, a portion of the grease that has reached the lateral surface 98b flows radially outward (i.e., rightward in FIG. 6) and is allowed to remain in the annular space 93. As indicated by the arrow A2 in FIG. 6, the other portion of the grease that has reached the lateral surface 98b may eventually pass through the labyrinth clearance 98a, resulting in leakage of the grease to the outside.
The locations of the angular contact ball bearings 90A and 90B used in a pair may be different from those illustrated in FIG. 5. The locations of the lower angular contact ball bearing 90A and the upper angular contact ball bearing 90B illustrated in FIG. 5 may be reversed. In this case, stopping the rotation of the inner ring 91 of the upper angular contact ball bearing 90A causes the grease in the annular space 93 to flow along the shoulder 96 of the inner ring 91, so that the grease is likely to pass through the labyrinth clearance 97a and leak to the outside.